David E. Steitz
Headquarters, Washington, DC July 12, 2001
(Phone: 202/358-1730)
Cynthia M. O'Carroll
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/614-5563)
RELEASE: 01-141
GREATER SOLAR ACTIVITY MAY BRING U.S. MORE GRAY DAYS
NASA-funded Earth Science researchers have discovered
that during periods of increased solar activity much of the
United States becomes cloudier, possibly because the jet
stream in the troposphere moves northward causing changes to
regional climate patterns.
The new study supports earlier findings by suggesting there
is a relationship between increased cloud cover over the
United States and the solar maximum, the most intense stage
of activity on the Sun.
Previous studies have shown that during the solar maximum,
the jet stream in the Northern Hemisphere moves northward.
The jet stream guides storms and plays an important role in
cloudiness, precipitation and storm formation in the United
States.
Dr. Petra Udelhofen, a NASA-funded researcher at the
Institute for Terrestrial and Planetary Atmospheres at the
State University of New York at Stony Brook, is the lead
author of a paper that discusses this topic, appearing in the
July 1 issue of Geophysical Research Letters.
"Based on these results and because the location of the jet
stream influences cloudiness," said Udelhofen, "we suggest
that the jet stream plays an important role in linking solar
variability and cloud cover."
The jet stream is a ribbon of fast-moving air in the upper
troposphere that blows from west to east. Storms beneath the
jet stream follow its path. A shift in the jet stream can
alter the location of clouds and precipitation across the
U.S.
The troposphere is the region of the atmosphere that extends
from the Earth's surface out to about 50,000 feet and is the
focus of local, regional and global weather research. The
stratosphere extends above the troposphere to about 150,000
feet and is the region where the ozone layer is formed.
The Sun's energy output varies over an 11-year cycle, sending
more ultraviolet radiation towards the Earth during times of
increased activity. While the Sun's total energy output only
varies by about one-tenth of one percent between periods of
low and high solar activity, the ultraviolet radiation that
affects ozone production in the stratosphere can change by
more than 10 percent.
Ultraviolet radiation is absorbed in the Earth's stratosphere
and creates the protective ozone layer. When the ozone
absorbs ultraviolet radiation, it warms the stratosphere,
which may affect movement of air in the troposphere where
clouds form.
Solar cycle effects of ultraviolet radiation absorption by
ozone in the stratosphere, its impact on atmospheric
circulation and the location of storm tracks have been the
subject of recent Earth Science research.
"Our results show that cloudiness varies on average by about
two percent between years of solar maximum and minimum. In
most parts of the U.S., cloud cover is slightly greater in
years of solar maximum," noted Udelhofen.
Though more investigation is needed to better understand just
how changes in the Sun's ultraviolet energy output is linked
to atmospheric winds, the study helps people identify
potential large-scale mechanisms that affect local and
regional climates.
Scientists continue to investigate mechanisms that may link
solar variability with weather. These new results support the
idea of a link between stratospheric chemistry and
meteorology, and support other recent theoretical studies
associated with the impact of stratospheric chemistry on
climate change and weather.
"It is important for future studies to identify and explain
in detail the link between solar variability, ozone, the
atmospheric circulation and cloud cover," Udelhofen said.
This research is part of the NASA Earth Science Enterprise
program, which is dedicated to understanding how Earth is
changing and what consequences these changes have for life on
Earth.
More information is available on the Internet at:
http://www.gsfc.nasa.gov/topstory/20010712cloudcover.html
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